The combination of transcranial electromagnetic brain stimulation (TEBS) and electroencephalography (EEG) enables direct modulation of neuronal activity at different cortical areas while simultaneously measuring the resulting response. Recording EEG concurrently with TEBS, however, is technically challenging.This Thesis attempts to solve several methodological problems related to combining EEG with the transcranial magnetic stimulation (TMS) and transcranial alternating current stimulation (tACS) techniques. A major problem with TMS–EEG is that TMS easily activates cranial muscles, which results in large EEG disturbances that mask cortically evoked responses. This work demonstrates how to avoid muscle artifacts when recording the data, as well as how to suppress the remaining artifacts in the offline signal processing with minimal distortion in the signals of interest. The common approach to study the cortical effects of TMS is to concentrate on the average evoked EEG response. This approach fails to inform us of how TMS affects the ongoing brain activity. This Thesis introduces quantitative tools to measure the TMS-induced changes in the brain-activity dynamics. The results indicate that TMS shifts the brain to an unnatural state, which is manifested as facilitated EEG variability. TEBS–EEG signals often suffer from transient noise and artifact components that are present only in certain channels or trials. This Thesis shows how the disturbances can be automatically cleaned so that the collected data are optimally utilized and that the time used in signal processing is decreased. The presented approach is directly usable also with conventional EEG and, e.g., with magnetoencephalography data. Combining EEG with concurrent tACS is difficult because of the large tACS-induced EEG artifacts. The results presented in this Thesis indicate that the tACS-related EEG artifact can be successfully scaled down while preserving meaningful neuronal responses at the stimulation frequency. This Thesis includes significant methodological advances that help to measure neuronal EEG signals in the harsh electromagnetic environment during TMS and tACS. The presented work enables a more flexible and reliable use of TEBS–EEG to study new brain areas and cognitive processes. The new tools help to utilize the gathered data fully, decreasing both the data acquisition and analysis times. Consequently, TEBS–EEG becomes a more feasible tool for novel research paradigms, as well as for clinical applications.
|Translated title of the contribution||Menetelmiä hermostollisten aivosähkökäyrävasteiden havaitsemiseksi sähkömagneettisen aivostimulaation aikana|
|Publication status||Published - 2017|
|MoE publication type||G5 Doctoral dissertation (article)|
- transcranial magnetic stimulation
- transcranial alternating current stimulation
- artifact rejection
- noise cancellation